One of the major expenses in a telephone system is that of providing a line interface circuit for connecting each and every telephone line at a central office switching facility, or at a private branch exchange facility. In any practical telephone system, the line interface circuits are desirably inexpensive and essentially electrically robust. During those times when a telephone set is in use, the associated telephone line interface circuit provides an energizing direct current for the telephone set, via tip and ring leads of the telephone line. When the telephone set is in use, it is usually referred to as being OFF HOOK, and when the telephone set is not in use, it is referred to as being ON HOOK. The line interface circuit is also required to provide indication as to the telephone set being ON HOOK or OFF HOOK.
In a central office, each telephone line is typically routed from an outside environment toward a switching facility via a main distributing frame (MDF). The MDF is a convenient place to provide protection from high voltage and current transient energies, as are from time to time conducted by one or more of the telephone lines as the result of a utility power line cross or a lightning strike. Protection is usually provided by the well known means of gas tubes which are arranged to conduct currents associated with potentials in excess of a thousand volts to a heavy ground bus bar. Tip and ring leads of each telephone line extend from the MDF to one of the line interface circuits.
Each line interface circuit is provided by circuit board mounted components which includes tip and ring terminals for connection to a telephone line and power terminals for connection to a source for the energizing current, usually a central battery supply. The central battery supply is continually charged to a predetermined voltage by a battery charging apparatus, provided for that purpose. The battery charging apparatus is operated from utility power, but in the event of an interruption of the utility power the central battery supply is intended to continue to supply operating current for a limited time.
One example of a line interface circuit is disclosed in the U.S. Pat. No. 4,103,112 issued on Jul. 25, 1978 to V. V. Korsky, and titled "Telephone Line Circuit With Differential Loop Current Sensing And Compensation". A telephone line is terminated via the tip and ring terminals by tip and ring windings of a miniature transformer. Energizing direct current is conducted by 200 ohm tip and ring feed resistors connected in series between the battery supply and the tip and ring windings. In this example the feed resistors are used as a means for accurately determining the amount of direct energizing current being conducted via the transformer windings and the telephone line so that flux compensation can prevent the core of the transformer from saturating. Thus the transformer couples communications signals while being a barrier to substantially reduce coupling of high energy transient signals as may result from a lightning strike or a utility power line cross. Nevertheless secondary and compensating windings of the transformer are coupled with protection networks to further reduce exposure of onfollowing circuitry to such transient signals. The feed resistors are exposed to the full brunt of any transient signals, and are therefore usually carried on a separate substrate which is thermally isolated from the other line circuit components. The feed resistors are ideally arranged to either withstand transient signal occurrences or decisively fail in the event of such an occurrence, so the maintenance of line circuit functionality will not be plagued by marginal performance problems.
U.S. Pat. Nos. 4,484,032 and 4,514,595 issued on Nov. 20, 1984 and Apr. 30, 1985 to S. Rosenbaum et al, teach an "Active Impedance Transformer Assisted Line Feed Circuit" and "Active Impedance Line Feed Circuit". One of the functions of these line interface circuits is that of providing a required terminating impedance via active circuit apparatus. Particularly, tip and ring amplifiers are controlled in response to direct loop current and alternating differential current signals detected across tip and ring feed resistors, to generate the terminating impedance and to inject alternating current signals for transmission via the telephone line to the telephone set. In the second example, the expense of a transformer is eliminated. In the first example, a function of the transformer is not intended to include a reduction of transient energies. In both examples, this function is typically provided by a pair of diodes connected between the power terminals and each of the tip and ring terminals so that any transient energies traversing the tip and ring leads from the MDF are diverted to the power terminals.
As the line currents detected are used as indication of alternating current signals in the telephone line and indication of direct energizing current, the performance of the line interface circuit is contingent upon the values of the feed resistors being very closely matched. The operating characteristics are compromised in proportion to the tip and ring feed resistors becoming mismatched. Such mismatch may occur during the normal use of a line interface circuit, as one of the tip and ring feed resistors may age in response to unusual stress, as may be caused by transient electrical events such as a power line cross or a lightning strike, in association with the telephone line, in spite of the diode protection at the tip and ring terminals.
In some instances other circuits in the typical electronic telephone switching office have been found to be at risk of failure particularly in the event of a wide spread transient electrical event. A nearby lightning strike upon a telephone cable, may simultaneously affect a multitude of telephone lines terminated in the switching office. In such an instance, a corresponding multitude of diode networks each couples a spike of current via the power terminals. These simultaneous current spikes are additive along a main power bus in the switching system and may cause a damaging transient variation of voltages on power and ground buses in parts of the switching facility.
It is one object of the invention to provide a line interface circuit with tip and ring feed paths which maintain preferred operating resistances in spite of transient electrical events which could be expected to adversely affect the normal operating performance of the line interface circuit.
It is also an object of the invention to provide line interface circuits which are electrically robust and provide substantial isolation of power and ground buses from transient electrical events in a telephone system.